CN112005159B

CN112005159B - Light modulator

Info

Publication number: CN112005159B
Application number: CN201980020020.6A
Authority: CN
Inventors: 川上广人; 山崎裕史; 宫本裕
Original assignee: Nippon Telegraph and Telephone Corp
Current assignee: Nippon Telegraph and Telephone Corp
Priority date: 2018-03-27
Filing date: 2019-03-19
Publication date: 2024-05-03
Anticipated expiration: 2039-03-19
Also published as: JP2019174560A; WO2019188597A1; US20210096440A1; CN112005159A; US11194219B2; JP6813528B2

Abstract

An optical modulator according to an embodiment includes: a first MZI and a second MZI each having a first optical coupler for 2-branching CW light, a second optical coupler for coupling and outputting the CW light branched by the first optical coupler, and a bias electrode for adjusting the phase of the CW light branched by the first optical coupler; a third optical coupler for coupling and outputting the outputs of the first and second MZIs at a predetermined ratio; and a bias adjustment circuit for adjusting the output voltage of the bias power supply applied to the bias electrode so that the optical path length difference of the CW light split by the first optical coupler becomes a predetermined multiple of the carrier wavelength under the condition that the output of the differential output amplifier is zero level according to the operation mode of the device.

Description

Light modulator

Technical Field

The present invention relates to a technique for improving the linearity between a data signal and the amplitude of an optical electric field or between the data signal and the light intensity in an optical transmitter using a Mach-zehnder type interferometer (MZI: mach-Zehnder interferometer).

Background

In a high-speed large-capacity optical transmission system, how to transmit a large amount of data per unit time becomes important. There have been actively studied a wavelength multiplexing technique for simultaneously transmitting signals of a plurality of wavelengths in the same optical fiber, a spatial multiplexing technique for simultaneously transmitting a plurality of signals using a plurality of cores arranged in the same optical fiber, or a technique for realizing high-speed and large-capacity optical transmission by combining these.

Whichever of these techniques is used, modulation of its light intensity or optical field is required to be applied at a certain wavelength as a carrier to generate an optical modulation signal. In order to generate a high-speed optical modulation signal, a CW (Continuous Wave) light source generating light without modulation and an optical modulator connected to an output side of the CW light source are generally used in combination. Although there are various types of optical modulators, MZI-type optical modulators capable of high-speed operation are widely used.

In the conventional optical modulation signal, only the on-state or the off-state of light represents the 2-value state of the 2-value, so that it corresponds to 1 and 0 of the bit. However, in recent years, in order to increase the capacity, multi-value modulation in which n bits are represented by modulating the light intensity or the amplitude of the optical electric field with a value of 2 ⁿ (n is a natural number) has become mainstream. For example, in the case of n=3, the value of 8 (=2 ⁿ) is set to correspond to (0, 0), (0, 1), (0, 1, 0), (0, 1), (1, 0, 1), (1, 0), (1, 1), this makes it possible to represent information of 3 bits by a carrier wave of a single wavelength.

Here, the terms "amplitude of the optical electric field" and "light intensity" will be described. Although the state of the optical modulation signal modulated with the digital signal varies according to the symbol rate, the optical modulation signal can be regarded as CW light if it is within a sufficiently short time than the symbol period. The optical field E of CW light can be expressed by the following expression (1), assuming that the frequency of CW light is f, the time is t, and the initial phase of light is Φrad.

[ Mathematics 1]

Here, E _A denotes the amplitude of the optical electric field. Here, the right side of the formula (1) is written as the following formula (2). Therefore, hereinafter, there is a case where the phase is changed by an amount of pi due to propagation delay on the modulator or the optical waveguide path, which is expressed as "sign inversion of the amplitude E _A of the optical electric field".

[ Math figure 2]

While which symbol is considered positive depends on the definition, in the optical modulation signal the relative change in phase of the light is important, and which definition is positive is not important.

In CW light, although the light intensity I is proportional to the square of the amplitude E _A of the optical electric field, in modulated light E _A is also modulated according to the symbol rate. Although the light intensity averaged over a sufficiently long period of time is sometimes expressed as "light intensity of modulated light", hereinafter, the light intensity I is assumed to represent the instantaneous intensity averaged over a sufficiently short period of time. Therefore, the value of the light intensity I in the following description means a value that always varies according to the symbol rate.

In the case of modulating the magnitude of the light intensity I or the optical electric field E _A with 2 ⁿ values, 2 ⁿ -1 thresholds are set, and although the symbol is distinguished by determining the magnitude relation of these thresholds and the light intensity or the optical electric field, the intervals of the thresholds are desirably equally spaced. This is because, when the interval between two thresholds is narrow, transmission errors due to noise are likely to occur in the symbol in the modulation state corresponding to the threshold. In addition, when the intervals of the thresholds are different, a complicated identification circuit corresponding to the thresholds having different intervals is required in the receiver, and design becomes difficult.

When the intervals between the thresholds are equal, attention must be paid to the nonlinearity of the optical modulator. In the case of performing multi-level modulation with an MZI-type optical modulator, in general, a multi-level electrical signal is amplified to generate a drive signal, and the drive signal is supplied to the MZI-type optical modulator to generate a multi-level optical modulation signal, but in general, the voltage of the drive signal is not linearly proportional to the light intensity I of the optical modulation signal or the amplitude E _A of the optical electric field. The reason for this will be described below with reference to fig. 9 to 13.

Fig. 9 is a diagram showing a configuration example of a conventional MZI-type optical modulator 90. The CW light output from the CW light source is split into two systems by the first optical coupler C1. Here, one is referred to as a P-side arm, and the other is referred to as an N-side arm. The P-side arm MP and the N-side arm MN are provided with a P-side drive signal electrode RP and an N-side drive signal electrode RN, respectively.

The P-side drive signal electrode RP and the N-side drive signal electrode RN change the phase of the CW light propagating through the P-side arm MP or the N-side arm MN according to the voltages applied to the electrodes. Although the phase advance or the phase delay differs depending on the structure of the modulator, the phase is retarded when a positive voltage is applied and advanced when a negative voltage is applied.

In general, in MZI-type optical modulators, a bias electrode is disposed to finely adjust the optical path length of the P-arm or the N-arm. Fig. 9 shows an example in which the bias electrode RB is disposed on the P-arm MP. The bias voltage is applied to the bias electrode RB by the bias power supply PS, and the phase of the CW light propagating through the P-arm is finely adjusted.

The P-side arm and the N-side arm are combined by a second optocoupler C2. Here, for the purpose of the following description, the second optical coupler C2 is a two-input two-output type coupler. The configuration of the two ports on the output side as optical paths is the same, but for reasons described later, one will be referred to as an output port and the other will be referred to as an inverted output port.

Next, a driving system of the MZI-type optical modulator 90 will be described. The electric signal supplied as a data signal to the MZI-type optical modulator 90 is amplified by the differential output amplifier AM and is input to the P-side drive signal electrode RP and the N-side drive signal electrode RN. The differential output amplifier AM outputs the amplified electric signal to one of the drive signal electrodes, and outputs an electric signal whose sign is inverted to the other drive signal electrode. The two data signals output have the same amplitude and on the other hand have waveforms that are inverted with respect to each other. Hereinafter, these two data signals are collectively referred to as "drive signals".

The driving signal is applied to the electrode RP for P-side driving signal and the electrode RN for N-side driving signal. When the voltage applied to the electrode RP for P-side drive signal is Vp, the voltage applied to the electrode RN for N-side drive signal is-Vp. Here, a value obtained by subtracting the voltage applied to the N-side drive signal electrode RN from the voltage applied to the P-side drive signal electrode RP is defined as "drive signal voltage". That is, in the previous example, the driving signal voltage is 2Vp. Since Vp can take a value of both positive and negative directions, the phase of light propagating through the P-side arm and the N-side arm is modulated in a push-pull manner with a driving signal.

Since the differential output amplifier AM is required to operate at a high speed, the frequency band of the differential output amplifier AM is generally a wide frequency band, but signals in the vicinity of a DC (Direct Current) component are generally blocked in terms of circuit configuration, and thus the drive signal voltage varies positively and negatively around zero (GND). Hereinafter, the amplitude of the fluctuation of the drive signal voltage is referred to as "drive amplitude".

The optical path length difference between the P-arm MP and the N-arm MN can be changed by the drive signal voltage or the bias voltage. In general, the half-wavelength voltage means a variation amount of voltage required to vary the optical path length difference by half the wavelength of CW light, and is denoted by vpi. Since vpi in the drive signal voltage is generally different from vpi in the bias voltage, the former is hereinafter referred to as vpi _DRIVE and the latter is hereinafter referred to as vpi _BIAS. In addition, although the driving signal may be applied to only one side of the driving signal electrode instead of the push-pull system due to the signal format or the device configuration, the present invention is not directly related to the problem to be solved, and therefore, the description thereof will be omitted.

Fig. 10 is a diagram schematically showing light output from an output port (P _N shown in fig. 9) of the MZI-type optical modulator 90 of the conventional structure. Fig. 10 (a) to (E) each show light output when the drive signal voltages are different. The solid line represents light passing through the P-side arm (hereinafter referred to as "light P"), and the broken line represents light passing through the N-side arm (hereinafter referred to as "light N"). The difference between t ₂ and t ₀ in the figure corresponds to the inverse of the optical frequency of the CW light (no relation to the symbol rate).

Fig. 10 (C) shows light P and light N in the case where the drive signal voltage is 0. The phase difference between the two is determined by the difference between the optical path length of the P-arm and the optical path length of the N-arm, but here, the phase difference is finely adjusted by the bias voltage as follows: the value of subtracting the phase of the light N from the phase of the light P in the case where the drive signal voltage is 0 is +pi/2 [ rad ]. The phase difference is 0.25 lambda if converted to carrier wavelength lambda. Hereinafter, the case where such a bias voltage is applied may be expressed as "bias the phase difference to +pi/2" or "bias the optical path length difference to +0.25λ".

Fig. 10 (D) and 10 (E) show light P and light N in the case where the voltage of the drive signal is positive. In this case, since the phase of the light P is retarded and the phase of the light N is advanced, the phase difference between the two decreases and the light intensity of the output port increases. In particular, when the drive signal voltage is +vρ _DRIVE/2 (fig. 10 (E)) phases coincide, and therefore the absolute value of the amplitude E _A of the optical field of the interference light in the output port becomes maximum, and the light intensity I also becomes maximum.

Fig. 10 (a) and 10 (B) show light P and light N in the case where the voltage of the drive signal is negative. In this case, since the phase of the light P advances and the phase of the light N delays, the phase difference between the two increases, and the light intensity of the output port decreases. In particular, when the drive signal voltage is-vρ _DRIVE/2 (fig. 10 (a)), phases are opposite to each other, and thus the amplitude E _A of the optical field of the interference light in the output port becomes zero, the light intensity I becomes zero as well, and the light disappears.

In the range of the drive amplitude voltage shown by fig. 10, the phases of light are controlled in a push-pull manner on the P side and the N side, and therefore the intensity of interference light in the output port P _N always becomes the valley side at t ₂、t₀ and always becomes the peak side at t ₁. The phase of the interfering light is always fixed. On the other hand, outside the range shown by fig. 10 (for example, in the case where the drive signal voltage is slightly smaller than-V pi _DRIVE/2), t ₂ and t ₀ and t ₁ are peaks Gu Fanzhuai of the waveforms, and thus the phase is changed by the amount of pi. As described above, this corresponds to the sign inversion of the amplitude E _A of the optical field.

That is, in order to generate the light intensity modulation signal using the conventional MZI-type optical modulator 90 shown in fig. 9, the optical path length difference is set to +0.25λ, and the upper limit and the lower limit of the drive signal voltage are set to +vρ _DRIVE/2 and-vρ _DRIVE/2, respectively. In this case, the driving amplitude becomes vpi _DRIVE, and if the driving amplitude exceeds the driving amplitude, a turn-back occurs in the light intensity I. The foldback referred to herein means that the light intensity is changed from increasing to decreasing or from decreasing to increasing.

Fig. 11 is a graph showing the amplitude E _A and the light intensity I of the optical electric field output from the output port P _N as a function of the drive signal voltage. The dashed line represents the amplitude E _A of the optical electric field and the solid line represents the light intensity I. The horizontal axis represents the drive signal voltage normalized by vpi _DRIVE, and the vertical axis represents the amplitude and light intensity of the optical electric field. For simplicity, fig. 11 omits a scaling factor of the amplitude and the light intensity of the optical electric field, which is not essential for the description. The amplitude E _A of the optical electric field does not have linearity with respect to the drive signal voltage, and performs a sine wave response. The light intensity I is proportional to the square of the amplitude of the optical electric field, but also has no linearity with respect to the drive signal voltage since the square of the sine wave is also a sine wave.

Although the light output from the output port P _N shown in fig. 9 is described above, the light output from the other inverted output port P _R shows a different optical field from the light output from the output port P _N even if the drive signal voltage and the bias voltage are the same. This is because the difference in phase of the light P and the phase of the light N differs by an amount of pi in the output port P _N and the inverted output port P _R. That is, in the output port P _N and the inverted output port P _R, the intensity of the output light varies inversely. Therefore, when the intensity of the output light of the output port P _N is maximum, the intensity of the output light of the inverted output port P _R becomes minimum, and the magnitude relationship is inverted.

Here, light output from the output port P _N in fig. 9 is considered again. In order to output a 4-value light intensity modulation signal (for example, a 4-value PAM (Pulse-Amplitude Modulation) modulation signal) from the output port P _N, a 4-value data signal may be amplified to generate a 4-value drive signal. As an example, the 4 value of the drive signal can be selected to be d4= +vpi _DRIVE/2、D3=+Vπ_DRIVE/6、D2=-Vπ_DRIVE/6、D1=-Vπ_DRIVE/2.

Fig. 12 shows a specific example of 4 levels L1 to L4 of light intensity obtained at this time. In this example, since the interval between L1 and L2 and the interval between L3 and L4 are narrower than the interval between L2 and L3, when the SNR (Signal to Noise Ratio: signal-to-noise ratio) after transmission is low, a decision error between L1 and L2 and between L3 and L4 often occurs.

Therefore, it is desirable that the 4 levels L1 to L4 are equally spaced. For this reason, a technique is known in which the drive amplitude (=d4—d1) is suppressed to be small and each interval is made close to an equal interval using a region where the linearity of the sine wave is good. In this method, although errors are not concentrated on a specific symbol, there is a problem that the error rate increases as a whole because the interval of the threshold value when the level is determined is narrowed in all the symbols.

The above description has been about the intensity modulation of light, but the multi-value modulation of the amplitude E _A of the optical electric field is also widely performed. For example, in optical QAM (Quadrature Amplitude Modulation:quadrature amplitude modulation), a plurality of MZI-type optical modulators are combined into a nested type, and the amplitude E _A of an optical electric field is subjected to multi-value modulation for each of two carriers having orthogonal phases. In this case, a plurality of levels including a negative value are set in the amplitude of the optical electric field. As described above, the negative optical field is realized by inverting the phase. Even In an I-Q (In-phase-quadrature) modulator that is widely used as a generator of an optical QAM signal at present, a plurality of conventional MZI-type optical modulators as shown In fig. 9 are combined to modulate the amplitude E _A of an optical electric field.

The modulation of the amplitude E _A of the optical electric field can be performed by biasing the optical path length difference to 0.5λ instead of 0.25λ (sometimes also denoted as "biasing to the null point") and setting the upper and lower limits of the drive signal voltage to +vρ _DRIVE and-vρ _DRIVE, respectively. Fig. 13 is a diagram showing a specific example of the light intensity I or the amplitude E _A of the optical electric field in this case.

The maximum value of the driving amplitude is 2V pi _DRIVE, and when exceeding this, a fold back occurs in the amplitude E _A of the optical electric field. As described above, it is understood that the amplitude E _A of the optical electric field also responds to the driving signal voltage in a sine wave manner like the light intensity I, and thus a problem of nonlinearity still occurs. Such a problem can be solved by an optical modulator that responds linearly to the drive signal voltage by using the light intensity I of the outputted light or the amplitude E _A of the optical electric field.

Such an optical modulator cannot be realized by a simple MZI-type optical modulator as shown in fig. 9, but a method of improving linearity by a combination of optical modulators having a more complex structure has been proposed. For example, non-patent document 1 proposes a light modulator capable of improving linearity of light intensity I with respect to a drive signal voltage, and non-patent document 2 and patent document 1 propose modulators capable of improving linearity of amplitude E _A of a light electric field with respect to a drive signal voltage.

Prior art literature

Patent literature

Patent document 1: international publication No. 2014/050123

Non-patent literature

Non-patent document 1: s, li, etc. for , "Highly linear radio-over-fiber system incorporating a single-drive dual-parallel Mach-Zehnder modulator," Photon. Technol. Lett., vol. 22, pp. 1775-1777, 2010 years for 12 months;

Non-patent document 2: H. yamazaki et al ,"Optical Modulator With a Near-Linear FieldResponse," J. Lightw. Technol., vol. 34, no. 16, pp. 3796-3801, 2016, 8.

Disclosure of Invention

Problems to be solved by the invention

However, the above-described conventional technique is specific to either improvement of the linearity of the amplitude E _A of the optical electric field or improvement of the linearity of the light intensity I, and therefore has a problem that both cannot be simultaneously achieved.

As for the structure of the optical circuit inside the optical modulator, once it is fabricated, the subsequent change is often difficult. Therefore, flexible use is required: an optical modulator having a structure described in non-patent document 1 or the like is used in a case where linearity of transmission light intensity becomes important (for example, a multi-valued PAM signal), and an optical modulator having a structure described in non-patent document 2 or patent document 1 or the like is used in a case where linearity of amplitude of transmission light electric field becomes important (for example, a multi-valued QAM signal).

In view of the above, an object of the present invention is to provide an optical modulator capable of modulating transmission data with an appropriate driving signal according to a signal format of a transmission signal.

Solution for solving the problem

An aspect of the present invention is an optical modulator, comprising: a first MZI (Mach-Zehnder interferometer ) provided with: a first optical coupler for 2-branching the CW light for the inputted carrier wave; a first arm and a second arm respectively connected to two outputs of the first optical coupler; and a second optical coupler that, after coupling the first arm and the second arm, again branches 2 and outputs from a first output port and a second output port; a second MZI including: a third optical coupler that performs 2-division after inputting the light output from the second output port; a third arm and a fourth arm respectively connected to two outputs of the third optical coupler; and a fourth optical coupler coupled to the third arm and the fourth arm and then output to a fifth arm; a sixth arm connected to the first output port; an asymmetric optical coupler for coupling the fifth arm and the sixth arm and outputting; a first differential output amplifier that differentially amplifies an input data signal; a first driving signal electrode and a second driving signal electrode for controlling a phase of light propagating through the first arm and the second arm in a push-pull manner according to an output of the first differential output amplifier; a first bias electrode for adjusting a phase of at least one of light propagating through the first arm or light propagating through the second arm according to an output voltage of a first bias power supply; a delay circuit that delays a correction signal, which is a signal having the same or positive/negative inversion of the data signal; a second differential output amplifier that differentially amplifies the correction signal delayed by the delay circuit; a third driving signal electrode and a fourth driving signal electrode for controlling the phase of light propagating through the third arm and the fourth arm in a push-pull manner according to the output of the second differential output amplifier; a second bias electrode for adjusting a phase of at least one of the light propagating through the third arm and the light propagating through the fourth arm according to an output voltage of a second bias power supply; a third bias electrode for adjusting a phase of at least one of the light propagating through the sixth arm and the light propagating through the fifth arm according to an output voltage of a third bias power supply; a first gain adjustment circuit and a second gain adjustment circuit for adjusting the output amplitudes of the first differential output amplifier and the second differential output amplifier, respectively; and a first bias adjustment circuit, a second bias adjustment circuit, and a third bias adjustment circuit for adjusting output voltages of the first bias power supply, the second bias power supply, and the third bias power supply, respectively, as operation modes of the apparatus, one of a first operation mode in which linearity of light intensity of light output from the apparatus is increased and a second operation mode in which linearity of amplitude of a light electric field of the light output from the apparatus is increased can be selected, light output from the first output port and light output from the second output port are light whose light intensity varies inversely, the asymmetric coupler outputs light having a photoelectric field added after multiplying each of the light electric field output from the sixth arm and the light electric field output from the fifth arm by a predetermined ratio, the first bias adjustment circuit selects a first operation mode in which a differential optical path length difference between the first arm and the second arm becomes 0.25 times a wavelength under a condition that an output of the first differential output amplifier becomes zero level, and a second bias mode in which a differential optical path difference between the first arm becomes 0.25 times a first bias voltage becomes a condition in which an output of the differential optical path difference between the first arm becomes 0.5 times a condition that the differential optical path difference becomes an output between the first carrier and the second arm becomes 0.

In one aspect of the present invention, the optical modulator is configured such that the second bias adjustment circuit adjusts the output voltage of the second bias power supply so that the optical path length difference between the third arm and the fourth arm approaches (0.5+0.05) or (0.5-0.05) times the carrier wavelength when the output of the second differential output amplifier is zero level when the first operation mode is selected, and adjusts the output voltage of the second bias power supply so that the optical path length difference between the third arm and the fourth arm approaches 0.5 times the carrier wavelength when the output of the second differential output amplifier is zero level when the second operation mode is selected.

In the optical modulator according to the aspect of the present invention, when a voltage difference between the applied voltage to the first Drive signal electrode and the applied voltage to the second Drive signal electrode is defined as Vdrive1, a change amount of Vdrive1 required for changing the interference intensity from the maximum to the minimum in the first output port, that is, the change amount of the half wavelength voltage in the Drive signal is defined as vpi 1, a voltage difference between the applied voltage to the third Drive signal electrode and the applied voltage to the fourth Drive signal electrode is defined as Vdrive2, a change amount of Vdrive2 required for changing the interference intensity from the maximum to the minimum in the fifth arm, that is, the change amount of the half wavelength voltage in the Drive signal is defined as vpi Drive2, the first gain adjustment circuit is controlled so that an absolute value of a difference between output voltages of the first differential output amplifier is not more than 0.7v×1 in the case of selecting the first operation mode, a differential difference between output voltages is not more than 0.7v×1 in the case of selecting the second operation mode, and a differential difference between output voltages of the first differential amplifier is not controlled so that the absolute value of the first differential amplifier is not more than 0.v×2 in the case of selecting the absolute value of the second differential amplifier is controlled so that the absolute value of the difference between the output modes is not more than the absolute value of the positive and negative.

An aspect of the present invention is an optical modulator, comprising: a fifth optical coupler for 2-branching the CW light for the inputted carrier; a seventh arm and an eighth arm respectively connected to two outputs of the fifth optical coupler; a first modulator connected to the seventh arm for modulating an I signal In an In-phase-quadrature (IQ) modulation; a second modulator connected to the eighth arm, for modulating a Q signal In IQ (In-phase-quadrature) modulation; an orthogonal control bias electrode for adjusting a phase of at least one of the light propagating through the seventh arm or the light propagating through the eighth arm according to an output voltage of an orthogonal control bias power supply; and a sixth optical coupler coupling the seventh arm and the eighth arm and outputting, the first modulator and the second modulator being the optical modulator of any one of the above.

Effects of the invention

According to the present invention, transmission data can be modulated with an appropriate drive signal according to the signal format of the transmission signal.

Drawings

Fig. 1 is a diagram showing a specific example of the structure of an optical modulator 1 according to the first embodiment.

Fig. 2 is a diagram showing a specific example of light input to the fifth optical coupler 32 when the optical path length difference between the P-side arm and the N-side arm of the main MZI 10 is biased to +0.25λ.

Fig. 3 is a diagram showing a specific example of light input to the fifth optical coupler 32 when the optical path length difference between the P-side arm and the N-side arm of the main MZI 10 is biased to +0.5λ.

Fig. 4 is a diagram showing a specific example of an eye pattern and a histogram of light intensity obtained when improvement of linearity of light intensity I is selected in the light modulator 1 of the first embodiment.

Fig. 5 is a diagram showing a specific example of an eye pattern and a histogram of light intensity obtained in an MZI-type optical modulator of a conventional structure.

Fig. 6 is a diagram showing an example of the method of extracting parameters in an operation mode in which the linearity of the light intensity I is increased and an operation mode in which the linearity of the amplitude E _A of the optical electric field is increased.

Fig. 7 is a diagram showing a specific example of the structure of the optical modulator 1a in the second embodiment.

Fig. 8 is a diagram showing a specific example of the bias of the main MZI and the bias of the correction MZI in the first linear optical modulator 62 and the second linear optical modulator 63.

Fig. 9 is a diagram showing a configuration example of a conventional MZI-type optical modulator 90.

Fig. 10 is a diagram schematically showing light output from an output port of the MZI-type optical modulator 90 of the conventional structure.

Fig. 11 is a graph showing the amplitude E _A and the light intensity I of the optical electric field output from the output port of the MZI-type optical modulator 90 of the conventional structure as a function of the drive signal voltage.

Fig. 12 is a diagram showing a specific example of the light intensity level obtained when a 4-value PAM modulation signal is output from the MZI-type optical modulator 90 having the conventional structure.

Fig. 13 is a diagram showing a specific example of the light intensity I and the amplitude E _A of the optical field in the case where the upper limit and the lower limit of the drive signal voltage are set to +vρ _DRIVE and-vρ _DRIVE, respectively, in the MZI-type optical modulator 90 of the conventional structure.

Detailed Description

First embodiment

Fig. 1 is a diagram showing a specific example of the structure of an optical modulator 1 according to the first embodiment. The optical modulator 1 of the first embodiment is similar to a conventional optical modulator (described in patent document 1, for example) in that it includes a main MZI 10 and a correction MZI 20, but is different from the conventional optical modulator in that it includes a first gain adjustment circuit 51, a second gain adjustment circuit 53, and a second bias adjustment circuit 54 as follows: the first gain adjustment circuit 51, the second gain adjustment circuit 53, and the second bias adjustment circuit 54 have a function of changing the driving amplitude and the bias voltage according to the case where the linearity of the amplitude E _A of the optical electric field is emphasized and the case where the linearity of the light intensity I is emphasized.

The main MZI 10 is basically the same as the MZI (Mach-Zehnder interferometer, mach-zehnder type interferometer) type optical modulator 90 of the conventional structure shown in fig. 9. The main MZI 10 includes a first optocoupler 11, a first P-side drive signal electrode 12P, a first N-side drive signal electrode 12N, a first bias electrode 13, and a second optocoupler 14. Like the optical modulator of the conventional structure, the second optical coupler 14 has an output port P _N and an inverted output port P _R. In the output port P _N and the inverted output port P _R, the intensity of the output light varies inversely.

The correction MZI 20 is connected to either one of the output port P _N or the inverted output port P _R of the main MZI 10, and corrects the modulated light output from the main MZI 10. In the first embodiment, it is set that the correction MZI 20 is connected to the inverted output port P _R of the main MZI 10.

The correction MZI 20 includes a third optocoupler 21, a second P-side drive signal electrode 22P, a second N-side drive signal electrode 22N, a second bias electrode 23, and a fourth optocoupler 24. The modulated light inputted to the correction MZI 20 is split into two systems of a P-side arm and an N-side arm by the third optical coupler 21. The second P-side drive signal electrode 22P is disposed on the P-side arm, and the second N-side drive signal electrode 22N is disposed on the N-side arm.

The second P-side drive signal electrode 22P and the second N-side drive signal electrode 22N change the phase of the modulated light propagating through the P-side arm and the N-side arm according to the drive signal voltage applied to the correction MZI 20. In the first embodiment, the second P-side drive signal electrode 22P and the second N-side drive signal electrode 22N are set to have a phase delayed by a positive voltage and a phase advanced by a negative voltage, similarly to the main MZI 10. In addition, in the correction MZI 20, the drive signal is applied in a push-pull manner, similarly to the main MZI 10.

In addition, a second bias electrode 23 is arranged at the rear stage of the second P-side drive signal electrode 22P on the P-side arm of the correction MZI 20. The second bias electrode 23 is applied with a bias voltage for correcting the MZI 20, thereby finely adjusting the phase of the modulated light propagating through the P-arm.

The P-arm and the N-arm are coupled by a fourth optical coupler 24, and the modulated light propagating through the respective arms is multiplexed (coupled) in the fourth optical coupler 24. The multiplexed modulated light is output from the correction MZI 20 to the correction signal arm. The correction signal arm is an arm connecting the fourth optocoupler 24 and the fifth optocoupler 32.

The third bias electrode 31 is connected to one of the output port P _N and the inverted output port P _R of the main MZI 10 to which the correction MZI 20 is not connected. That is, in the first embodiment, the third bias electrode 31 is connected to the output port P _N of the main MZI 10. The third bias electrode 31 is applied with a bias voltage by the third bias power supply 45, thereby correcting the phase of the modulated light output from the main MZI 10.

The fifth optocoupler 32 is a two-input one-output asymmetric optocoupler. An asymmetric optocoupler refers to an optocoupler that adds the photoelectric fields of two modulated lights inputted and outputs them. An asymmetric optocoupler has a power supply with 1: and the ratio addition of X. The asymmetric optocoupler is also capable of equally adding the input optical fields. The fifth optical coupler 32 receives the modulated light outputted from the correction MZI 20 and propagating through the correction signal arm and the modulated light propagating through the main signal arm and passing through the third bias electrode 31. The fifth optical coupler 32 combines and outputs the inputted modulated light at a predetermined ratio. The main signal arm is an arm connecting the output port P _N and the fifth optical coupler 32.

The driving system 40 includes a first differential output amplifier 41, a first bias power supply 42, a second differential output amplifier 43, a second bias power supply 44, and a third bias power supply 45. The second differential output amplifier 43 differentially amplifies the correction signal to generate a driving signal for correcting the MZI 20. The correction signal is the same as the data signal or a signal whose sign is inverted. In the first embodiment, the correction signal and the data signal are given the same sign. Further, since the DC component is generally blocked in the first differential output amplifier 41 and the second differential output amplifier 43 as described above, the correction signal and the data signal are positively and negatively swung around zero (GND level).

The control system 50 includes a first gain adjustment circuit 51, a first bias adjustment circuit 52, a second gain adjustment circuit 53, a second bias adjustment circuit 54, a delay circuit 55, and a third bias adjustment circuit 56. The first gain adjustment circuit 51 can change the gain of the first differential output amplifier 41. The first bias adjustment circuit 52 can change the bias voltage applied to the first bias electrode 13 of the main MZI 10 by the first bias power supply 42. The third bias adjustment circuit 52 can change the bias voltage applied to the third bias electrode 31 by the third bias power supply 45.

The second gain adjustment circuit 53 can change the gain of the second differential output amplifier 43. The second bias adjustment circuit 54 can change the bias voltage applied to the second bias electrode 23 of the correction MZI 20 by the second bias power supply 44.

The delay circuit 55 delays the correction signal by a prescribed time (hereinafter referred to as "delay time").

The delay time is set to be equal to the delay time of light from the first P-side drive signal electrode 12P or the first N-side drive signal electrode 12N (from point B or point C in fig. 1) to the second P-side drive signal electrode 22P or the second N-side drive signal electrode 22N (to point F or point G in fig. 1).

Next, a method of optimizing the linearity of the light intensity I in the light modulator 1 of the first embodiment will be described. Hereinafter, for simplicity, V pi _DRIVE in the driving signal voltage for the main MZI 10 and V pi _DRIVE in the driving signal voltage for the correction MZI 20 are equal, and both are described as V pi _DRIVE.

The main MZI 10 uses the first bias adjustment circuit 52 and the first bias power supply 42 to bias the optical path length difference between the P-side arm and the N-side arm in the main MZI 10 to +0.25λ. The light observed at point E of fig. 1 is shown in fig. 2 (a). This is the same as shown in fig. 11. Here, the extrinsic coefficients are also omitted. In addition, the horizontal axis represents the drive signal voltage normalized with V pi _DRIVE. The light intensity I at the point E becomes zero when the horizontal axis is-0.5, and the light intensity I at the point E becomes maximum value 1 when the horizontal axis is +0.5. In a region where the absolute value of the horizontal axis is 0.5 or less, the amplitude E _A of the optical electric field is always positive. However, as described above, since the light intensity is inverted in the inversion output port P _R, the light intensity I at the point D in fig. 1 becomes maximum value 1 when the horizontal axis is-0.5, and the light intensity I at the point D becomes zero when the horizontal axis is +0.5.

The correction MZI 20 uses the second bias adjustment circuit 54 and the second bias power supply 44 to bias the optical path length difference between the P-side arm and the N-side arm in the correction MZI 20 to +1.1λ/2=0.55λ. That is, when the driving signal voltage for the correction MZI is 0, the value obtained by subtracting the optical path length from the D point to the H point on the correction signal arm via the P side arm of the correction MZI 20 from the optical path length from the D point to the H point on the correction signal arm via the N side arm of the correction MZI 20 on the inversion output port P _R of the main MZI 10 is set to 0.55λ.

The light intensity I and the amplitude E _A of the optical field observed at point H of fig. 1 are shown in fig. 2 (B). It is understood that the sign of the amplitude E _A of the optical electric field is inverted when the driving signal voltage for the main MZI 10 and the driving signal voltage for the correction MZI 20 normalized by vpi _DRIVE are in the vicinity of +0.3 and in the vicinity of-0.3.

The optical field at the H point and the optical field at the E point are coupled by a fifth optocoupler 32, which is an asymmetric optocoupler, at about 1: the ratio of 0.4 is added (the ratio is the ratio of the optical electric field rather than the ratio of the intensity) and output from the modulator output port P _X. At this time, the voltage applied to the third bias electrode 31 by the third bias power supply 45 is set so as to maximize the interference efficiency of the fifth optocoupler 32.

When the driving signal voltage for the main MZI 10 and the driving signal voltage for the correction MZI 20 normalized by vpi _DRIVE are +0.3, the optical field at the E point and the optical field at the H point, which are opposite in phase to each other, are added by the fifth optocoupler 32, thereby suppressing the light intensity I.

When the driving signal voltage for the main MZI 10 normalized by vpi _DRIVE and the driving signal voltage for the correction MZI 20 are in the vicinity of-0.3, the optical field at the point E and the optical field at the point H, which are in the same phase, are added by the fifth optocoupler 32, thereby increasing the light intensity I.

The light intensity I and the amplitude E _A of the optical electric field in the modulator output port P _X are shown in fig. 2 (C). It is understood that the linearity of the light intensity I is improved compared with the linearity at the point E in the range of-0.5 to +0.5 for the driving signal voltage for the main MZI 10 normalized with vpi _DRIVE and the driving signal voltage for the correction MZI 20. It is understood that even if the range of the driving signal voltage for the main MZI 10 normalized by vpi _DRIVE and the driving signal voltage for the correction MZI 20 is set to-0.7 to +0.7, no turn-back occurs in the light intensity I. That is, it is understood that the degree of freedom in driving amplitude increases as compared with the MZI-type optical modulator 90 of the conventional structure shown in fig. 9.

The driving amplitude for the main MZI 10 is set by the first gain adjustment circuit 51 and the first differential output amplifier 41 to not exceed-0.7V pi _DRIVE～+0.7Vπ_DRIVE. The driving amplitude for correcting the MZI 20 is set to be the same as the driving amplitude for the main MZI 10 by the second gain adjustment circuit 53 and the second differential output amplifier 43.

Next, a method of optimizing the linearity of the amplitude E _A of the optical electric field in the optical modulator 1 of the first embodiment will be described. The main MZI 10 uses the first bias adjustment circuit 52 and the first bias power supply 42 to bias the optical path length difference between the P-side arm and the N-side arm in the main MZI 10 to +0.5λ (if represented more generally, to a null point).

The light observed at point E of fig. 1 is shown in fig. 3 (a). This is the same as shown in fig. 13. Here, the extrinsic coefficients are also omitted. In addition, the horizontal axis represents the drive signal voltage normalized with V pi _DRIVE. The amplitude E _A of the optical field at the E point becomes a minimum value, i.e., -1, when the horizontal axis is-1.0, and the amplitude E _A of the optical field at the E point becomes a maximum value, i.e., 1, when the horizontal axis is +1.0. The amplitude E _A of the optical electric field also becomes 0 when the horizontal axis is 0.

The correction MZI 20 uses the second bias adjustment circuit 54 and the second bias power supply 44 to bias the optical path length difference between the P-side arm and the N-side arm in the correction MZI 20 to 0.5λ (= +1.0λ/2). That is, when the driving signal voltage for the correction MZI 20 is 0, the value obtained by subtracting the optical path length from the D point on the inverting output port P _R of the main MZI 10 to the H point on the correction signal arm via the P-side arm of the correction MZI 20 from the optical path length from the D point to the H point on the correction signal arm via the N-side arm of the correction MZI 20 is set to 0.5λ.

The light intensity I and the amplitude E _A of the optical field observed at point H of fig. 1 are shown in fig. 3 (B). The sign of the amplitude E _A of the optical field is inverted when the driving signal voltage for the main MZI 10 normalized by vpi _DRIVE and the driving signal voltage for the correction MZI 20 are set to 0. Further, the sign is a sign opposite to the amplitude E _A of the optical electric field observed at the point E of fig. 1.

The optical field at point H and the optical field at point E are at about 1 by the fifth optocoupler 32: the ratios of 0.3 are added and output from the modulator output port P _X. At this time, the voltage applied to the third bias electrode 31 by the third bias power supply 45 is set so as to maximize the interference efficiency of the fifth optocoupler 32. The phases of the optical field at point E and the optical field at point H are always opposite (amplitude E _A is a negative sign) regardless of the drive signal voltage, and therefore the absolute value of amplitude E _A is suppressed by fifth optocoupler 32.

The light intensity I and the amplitude E _A of the optical electric field in the modulator output port P _X are shown in fig. 3 (C).

It is understood that in the range of-1 to +1 for the driving signal voltage for the main MZI 10 normalized with vpi _DRIVE and the driving signal voltage for the correction MZI 20, the linearity of the amplitude E _A of the optical electric field is improved compared with the linearity at point E.

The driving amplitude for the main MZI 10 is set to not exceed-vpi _DRIVE～+Vπ_DRIVE by the first gain adjustment circuit 51 and the first differential output amplifier 41. The driving amplitude for correcting the MZI 20 is set to be the same as the driving amplitude for the main MZI 10 by the second gain adjustment circuit 53 and the second differential output amplifier 43.

As described above, in the first embodiment, the linearity of the amplitude E _A of the optical electric field or the linearity of the light intensity I can be improved by changing the driving amplitude and the bias voltage without adding any change to the configuration of the optical circuit.

Fig. 4 shows the result of actual measurement of an eye pattern and a histogram of light intensity obtained by square detection of a 4-value light intensity modulation signal (4-value PAM) generated by selecting improvement in linearity of the light intensity I using the optical modulator 1 described in the first embodiment. Fig. 5 shows the result of the same measurement using the single MZI-type optical modulator 90 of the conventional structure shown in fig. 9. The drive amplitude is set to V pi _DRIVE. When comparing the two, in the measurement result (fig. 5) in the conventional configuration, as shown in fig. 12, the 4 light intensity levels are not equally spaced, and the interval between the two values in the center is expanded. In contrast, it is understood that the linearity improves in the measurement result (fig. 4) using the optical modulator 1 of the first embodiment.

For an operation example in the case of using the optical modulator 1 described in the first embodiment and selecting improvement of linearity of the amplitude E _A of the optical electric field, refer to non-patent document 2, patent document 1, and the like. However, in these prior art and the present invention, the coupling ratio of the asymmetric coupler behaves differently. In the first embodiment, the optical field at the point D and the optical field at the point E described in fig. 1 are set to be approximately 1: a ratio of 0.3 is added, but if rewritten at the ratio of intensities, becomes 1 ²：0.3² =1: 0.09 =1-0.083: since the light intensity combination ratio r used in patent document 1 is 0.083, the value of the light intensity combination ratio r is 0.083 in this example.

A modification of the first embodiment will be described below. In the first embodiment, it is assumed that the positive voltage is applied to the drive signal electrode to delay the phase, and the negative voltage is applied to advance the phase, but the opposite may be applied depending on the configuration of the modulator. In the first embodiment, the driving signal voltage for the main MZI 10 and the driving signal voltage for the correction MZI 20 are not logically inverted, but the first embodiment can be operated even if the driving signal voltage is inverted. In addition, other optimal solutions may exist in each bias voltage due to the periodicity of the main MZI 10 and the correction MZI 20.

Fig. 6 (a) shows an example of the method of extracting each parameter in the operation mode in which the linearity of the light intensity I is increased, and fig. 6 (B) shows an example of the method of extracting each parameter in the operation mode in which the linearity of the amplitude E _A of the optical electric field is increased.

The "main and corrected logic inversion" described in fig. 6 (a) shows the presence or absence of the inversion of the logic of the driving signal voltage for the main MZI 10 and the driving signal voltage for the corrected MZI 20. The "asymmetric coupler coupling ratio" is a ratio in which the optical electric field at the H point and the optical electric field at the E point are combined as described above. For example, "-1 to 0.4" described in fig. 6 (a) means that the modulated light propagating through the main signal arm MM of fig. 1 is given a delay for correcting the propagation delay time of the MZI 20 by the third bias power supply 45 and the third bias electrode 31, and is input to the asymmetric coupler after the phase is changed by pi [ rad ] (after a considerable delay corresponding to half the carrier wavelength is added if the wavelength is used). The negative sign is marked because the phase change of pi rad is equivalent to the sign inversion of the amplitude E _A of the optical field as described above.

The numerical values shown in fig. 6 are typical values, and are not necessarily limited to these values. For example, in the case of disregarding the nonlinear response of not only the optical modulator but also the first differential output amplifier 41 or the second differential output amplifier 43, it is possible to make a number of changes to the number and correct it so as to obtain the optimum linearity of the system as a whole.

In the above description, V pi _DRIVE of the main MZI 10 and V pi _DRIVE of the correction MZI 20 are the same. When the two are different, for example, in the case where V pi _DRIVE of the correction MZI 20 is Y times larger than V pi _DRIVE of the main MZI 10, the driving amplitude for the correction MZI 20 may be set to Y times the driving amplitude for the main MZI 10 so that the magnitude of the driving signal for each V pi _DRIVE is kept constant.

Second embodiment

Fig. 7 is a diagram showing a specific example of the structure of the optical modulator 1a in the second embodiment. The optical modulator 1a is a modulator for IQ modulation, and has a structure in which MZI-type optical modulators are combined in a nested manner. The optical modulator 1a includes a first optical coupler 61, a first linear optical modulator 62, a second linear optical modulator 63, an electrode 64 for orthogonal control bias, a power supply 65 for orthogonal control bias, and a second optical coupler 66. The first linear light modulator 62 and the second linear light modulator 63 are the same as the light modulator 1 of the first embodiment.

The CW light is split into two systems, I side arm and Q side arm, in a first optical coupler 61. The I side arm is connected to a first linear light modulator 62 and the q side arm is connected to a second linear light modulator 63.

The first linear optical modulator 62 and the second linear optical modulator 63 are driven by a data signal (I) and a data signal (Q), respectively. The output light of the first linear optical modulator 62 and the second linear optical modulator 63 is multiplexed by the second optical coupler 66 and output from the modulator output port P _X, but the phase difference between the two is adjusted by applying the output voltage of the quadrature control bias power supply 65 to the quadrature control bias electrode 64 and controlling the delay time.

In the second embodiment, either one of an operation mode (an example of the first operation mode) in which the linearity of the light intensity I is increased or an operation mode in which the linearity of the magnitude E _A of the optical electric field is increased may be selected.

First, a case will be described in which an operation mode in which the linearity of the amplitude E _A of the optical electric field is increased is selected to generate an optical QAM signal. The bias of the main MZI, the bias of the correction MZI in the first linear optical modulator 62 and the second linear optical modulator 63 may be the same as that shown in fig. 6 (B). The difference in optical path lengths of the I-arm and the Q-arm, which is observed from the modulator output port P _X, is adjusted by the quadrature control bias power supply 65 so as to be ±0.25λ when the drive signal voltage is at the zero level. This is a state in which the phases of the light propagating through the I-arm and the light propagating through the Q-arm are orthogonal, and the intensities of the two do not interfere with each other.

If the data signal (I) and the data signal (Q) are multi-valued signals of n values, respectively, an optical n ² -QAM signal is generated in the output port. Since the linearity of the amplitude E _A of the optical electric field is ensured by the first linear optical modulator 62 and the second linear optical modulator 63, n ² symbols are arranged at equal intervals in the generated constellation (constellation) even if the driving amplitude is set to be large.

Next, a case will be described in which an operation mode (an example of the second operation mode) in which the linearity of the light intensity I is increased is selected to generate the optical PAM signal. Fig. 8 shows the bias of the main MZI, the correction MZI, in the first linear optical modulator 62 and the second linear optical modulator 63 in this case. Further, the data signal (Q) is set to always zero level. The outputs of the first differential output amplifier and the second differential output amplifier (refer to fig. 1) included in the second linear optical modulator 63 are at zero level. The bias of the main MZI and the bias of the correction MZI of the first linear optical modulator 62 are biased to 0.5λ (biased to the null point) so that light propagating through the Q-side arm is extinction within the second linear optical modulator 63. The output voltage of the quadrature control bias power supply 65 may be any value. Since the light output from the output port is substantially the same as that of the first embodiment, a PAM signal having good linearity as shown in fig. 4 can be obtained in the optical modulator 1a of the second embodiment.

Although the embodiments of the present invention have been described above with reference to the drawings, specific configurations are not limited to the embodiments, and include designs and the like that do not depart from the scope of the present invention.

In the main MZI 10 shown in fig. 1, the P-side arm connected to the first optical coupler 11 is an example of the first arm in the present invention. The N-side arm connected to the first optical coupler 11 is an example of the second arm in the present invention. The first P-side drive signal electrode 12P is an example of the first drive signal electrode in the present invention. The first P-side drive signal electrode 12N is an example of the second drive signal electrode in the present invention. The first optical coupler 11 is an example of the first optical coupler in the present invention. The second optocoupler 14 is an example of the second optocoupler of the present invention. The output port on the P _N side of the second optocoupler 14 is an example of the first output port in the present invention. The output port on the P _R side of the second optocoupler 14 is an example of the second output port in the present invention.

In addition, in the correction MZI 20, the P-side arm connected to the third optical coupler 21 is an example of the third arm in the present invention. The N-side arm connected to the third optocoupler 21 is an example of the fourth arm in the present invention. The second P-side drive signal electrode 22P is an example of the third drive signal electrode in the present invention. The second P-side drive signal electrode 22N is an example of the fourth drive signal electrode in the present invention. The third optocoupler 21 is an example of the third optocoupler in the present invention. The fourth optocoupler 24 is an example of the fourth optocoupler of the present invention.

In the optical modulator 1, the correction signal arm is an example of the fifth arm in the present invention. The main signal arm is an example of the sixth arm in the present invention. In the optical modulator 1a, the I-side arm connected to the first optical coupler 61 is an example of the seventh arm in the present invention. The Q-side arm connected to the first optical coupler 61 is an example of the eighth arm in the present invention. The first linear optical modulator 62 is an example of the first modulator in the present invention. The second linear optical modulator 63 is an example of the second modulator in the present invention. The first optocoupler 61 is an example of the fifth optocoupler of the present invention. The second optocoupler 66 is an example of the sixth optocoupler of the present invention.

Industrial applicability

The present invention can be applied to an optical transmitter that modulates an optical signal to transmit data.

Description of the reference numerals

1,1A optical modulator, 10 main MZI (Mach-, mach-Zehnder interferometer), 11 first optocoupler, 12N first N side driving signal electrode, 12P first P side driving signal electrode, 13 first bias electrode, 14 second optocoupler, 20 correction MZI,21 third optocoupler, 22N second N side driving signal electrode, 22P second P side driving signal electrode, 23 second bias electrode, 24 fourth optocoupler, 31 third bias electrode, 32 fifth optocoupler, 40 driving system, 41 first differential output amplifier, 42 first bias power supply, 43 second differential output amplifier, 44 second bias power supply, 45 third bias power supply, 50 control system, 51 first gain adjustment circuit, 52 first bias adjustment circuit, 53 second gain adjustment circuit, 54 second bias adjustment circuit, 55 delay circuit, 56 third bias adjustment circuit, 61 first optocoupler, 62 first linear optical modulator, 63 second linear optical modulator, 64 orthogonally controlled bias electrode, 65 orthogonally controlled bias power supply, 66 second optocoupler, 90 MZI type optical modulator of conventional structure, AM differential output amplifier, MN N side arm, MP P side arm, PN output port, PR inverted output port, PS bias power supply, RB bias electrode, RN side drive signal electrode, RP side drive signal electrode.

Claims

1. An optical modulator, comprising:

A first MZI (Mach-Zehnder interferometer ) provided with: a first optical coupler for 2-branching the CW light for the inputted carrier wave; a first arm and a second arm respectively connected to two outputs of the first optical coupler; and a second optical coupler that, after coupling the first arm and the second arm, again branches 2 and outputs from a first output port and a second output port;

a second MZI including: a third optical coupler for performing 2-division after inputting the light outputted from the second output port; a third arm and a fourth arm respectively connected to two outputs of the third optical coupler; and a fourth optical coupler coupled to the third arm and the fourth arm and then output to a fifth arm;

a sixth arm connected to the first output port;

an asymmetric optical coupler for coupling the fifth arm and the sixth arm and outputting;

a first differential output amplifier that differentially amplifies an input data signal;

A first driving signal electrode and a second driving signal electrode for controlling a phase of light propagating through the first arm and the second arm in a push-pull manner according to an output of the first differential output amplifier;

A first bias electrode for adjusting a phase of at least one of light propagating through the first arm or light propagating through the second arm according to an output voltage of a first bias power supply;

A delay circuit that delays a correction signal, which is a signal having the same or positive/negative inversion of the data signal;

A second differential output amplifier that differentially amplifies the correction signal delayed by the delay circuit;

A third driving signal electrode and a fourth driving signal electrode for controlling the phase of light propagating through the third arm and the fourth arm in a push-pull manner according to the output of the second differential output amplifier;

a second bias electrode for adjusting a phase of at least one of the light propagating through the third arm and the light propagating through the fourth arm according to an output voltage of a second bias power supply;

a third bias electrode for adjusting a phase of at least one of the light propagating through the sixth arm and the light propagating through the fifth arm according to an output voltage of a third bias power supply;

A first gain adjustment circuit and a second gain adjustment circuit for adjusting the output amplitudes of the first differential output amplifier and the second differential output amplifier, respectively; and

A first bias adjustment circuit, a second bias adjustment circuit, and a third bias adjustment circuit for adjusting output voltages of the first bias power supply, the second bias power supply, and the third bias power supply, respectively,

As the operation mode of the present device, either one of a first operation mode in which the linearity of the light intensity of the light output from the present device is increased or a second operation mode in which the linearity of the amplitude of the optical electric field of the light output from the present device is increased can be selected,

The light output from the first output port and the light output from the second output port are light whose light intensity varies inversely,

The asymmetric coupler outputs light having a photoelectric field added after multiplying each of the photoelectric field output from the sixth arm and the photoelectric field output from the fifth arm by a prescribed ratio,

The first bias adjustment circuit adjusts the output voltage of the first bias power supply so that the optical path length difference between the first arm and the second arm becomes 0.25 times the carrier wavelength when the output of the first differential output amplifier is at the zero level when the first operation mode is selected, and adjusts the output voltage of the first bias power supply so that the optical path length difference between the first arm and the second arm becomes 0.5 times the carrier wavelength when the output of the first differential output amplifier is at the zero level when the second operation mode is selected.

2. The optical modulator according to claim 1, wherein the second bias adjustment circuit adjusts the output voltage of the second bias power supply so that the optical path length difference between the third arm and the fourth arm approaches (0.5+0.05) times or (0.5-0.05) times the carrier wavelength when the output of the second differential output amplifier is at the zero level in the case where the first operation mode is selected, and adjusts the output voltage of the second bias power supply so that the optical path length difference between the third arm and the fourth arm approaches 0.5 times the carrier wavelength when the output of the second differential output amplifier is at the zero level in the case where the second operation mode is selected.

3. The light modulator of claim 1 wherein,

When the voltage difference between the voltage applied to the first Drive signal electrode and the voltage applied to the second Drive signal electrode is defined as Vdrive1, the amount of change in Vdrive1 required to change the interference intensity in the first output port from maximum to minimum, that is, the amount of change in half-wavelength voltage in the Drive signal is defined as vpi Drive1, the voltage difference between the voltage applied to the third Drive signal electrode and the voltage applied to the fourth Drive signal electrode is defined as Vdrive2, the amount of change in Vdrive2 required to change the interference intensity in the fifth arm from maximum to minimum, that is, the amount of change in half-wavelength voltage in the Drive signal is defined as vpi Drive2,

The first gain adjustment circuit is controlled such that an absolute value of a difference between the forward and reverse output voltages of the first differential output amplifier does not exceed 0.7xvpi Drive1 in the case where the first operation mode is selected, and is controlled such that an absolute value of a difference between the forward and reverse output voltages of the first differential output amplifier does not exceed vpi Drive1 in the case where the second operation mode is selected,

The second gain adjustment circuit is controlled such that an absolute value of a difference between the forward and reverse output voltages of the second differential output amplifier does not exceed 0.7xvjdrive 2 when the first operation mode is selected, and is controlled such that an absolute value of a difference between the forward and reverse output voltages of the second differential output amplifier does not exceed vjdrive 2 when the second operation mode is selected.

4. An optical modulator, comprising:

a fifth optical coupler for 2-branching the CW light for the inputted carrier;

a seventh arm and an eighth arm respectively connected to two outputs of the fifth optical coupler;

A first modulator connected to the seventh arm for modulating an I signal In an In-phase-quadrature (IQ) modulation;

A second modulator connected to the eighth arm, for modulating a Q signal In IQ (In-phase-quadrature) modulation;

An orthogonal control bias electrode for adjusting a phase of at least one of the light propagating through the seventh arm or the light propagating through the eighth arm according to an output voltage of an orthogonal control bias power supply; and

A sixth optical coupler coupling the seventh arm and the eighth arm and outputting,

The first modulator and the second modulator are the optical modulators of any one of claims 1 to 3.